Capablanca.
Hardest to beat players

Thank you. Let's compare (www.chessgames.com):
- Petrosian 159 loses / 1938 games = 8 %
- Capablanca 49 loses / 818 games = 6 %
- Ulf Andersson 234 loses / 2311 games = 10 %
Who else ???

Petrosian. Anyone that can go 2 years in a row without losing a game in the USSR championship, is hard to beat.
Capablanca.
Well Kramnik didn't lose a single game to Kasparov in the 2000 world championship, so I'd say he probably qualifies :P

Ok, so we have (I've added Kramnik (and Karpov) and re-ordered players by the percentage of loses):
(1) Kramnik 139 loses / 2495 games = 5.6 %
(2) Capablanca 49 loses / 818 games = 6.0 %
(3) Karpov 235 loses / 3559 games = 7 %
(4) Petrosian 159 loses / 1938 games = 8 %
(5) Ulf Andersson 234 loses / 2311 games = 10 %

(1) Kramnik 139 loses / 2495 games = 5.6 %
(2) Capablanca 49 loses / 818 games = 6.0 %
(3) Karpov 235 loses / 3559 games = 7 %
(4) Petrosian 159 loses / 1938 games = 8.2 %
(5) Fischer 86 loses / 992 games = 8.7 %
(6) Ulf Andersson 234 loses / 2311 games = 10 %
Hello! What are (were) the hardest to beat players in history (i.e. the ones who not necessarily win often, but the ones who lose very rarely)? I will start with iron Tigran Petrosian.